Merge branch 'rmills/feature-ml-petscregressor' into 'main'

Add basic PetscRegressor component with support for a few linear regressors

See merge request petsc/petsc!7709

Add initial version PetscRegressor, part of the planned machine learning (ML) toolkit

Co-authored-by: Mr. Hong Zhang <hongzhang@anl.gov>
Co-authored-by: Hansol Suh <hsuh@anl.gov>
Co-authored-by: Bar

Add initial version PetscRegressor, part of the planned machine learning (ML) toolkit

Co-authored-by: Mr. Hong Zhang <hongzhang@anl.gov>
Co-authored-by: Hansol Suh <hsuh@anl.gov>
Co-authored-by: Barry Smith <bsmith@petsc.dev>
Co-authored-by: Toby Isaac <toby.isaac@gmail.com>
Co-authored-by: Satish Balay <balay@mcs.anl.gov>

Mr. Hong Zhang developed the petsc4py interface and examples.
Hansol Suh provided extensive help with updating the code to the latest PETSc coding guidelines and APIs, some documentation development, getting documentation to build correctly, getting new tests into the test harness, and numerous big fixes plus minor changes.
Barry Smith provided extensive feedback and code to get the Fortran interfaces and documentation build working.
Toby Isaac provided a fix to prevent breakage with complex-arithmetic builds.
Satish Balay provided important help with fixing builds.

Thanks-to: Pierre Jolivet <pierre@joliv.et>
Thanks-to: Jed Brown <jed@jedbrown.org>
Thanks-to: Jose Roman <jroman@dsic.upv.es>
Thanks-to: Stefano Zampini <stefano.zampini@gmail.com>

Pierre, Jed, Jose, and Stefano provided extremely helpful feedback and suggestions during the merge review process.

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